How to have sharp shadows in cycles with HDRI
When you use an HDRi as environment texture you are you are mapping an image in a giant sphere that surrounds the scene and emits light, so every pixel from the image contributes to the lighting in the scene. 

As explained in the comments the sharpness of the shadows is directly related to the size of the bright elements on the image. An HDRi of a sunny blue sky will give you a sharp shadow, an image of an overcast or shady environment will give you softer shadows. 
Other than controlling the intensity and placement of the image in the world, there are no built in tools to change the quality of the shadows.